In primates, separation of an infant from its mother is a naturally occurring stressor resulting in activation of behavioral, endocrine, and autonomic systems. When separated from their mothers, infant rhesus monkeys emit frequent species-typical distress vocalizations ('coos'). In earlier work we demonstrated that opiate and benzodiazepine systems influence the frequency of coos induced by separation in infant rhesus monkeys. The present studies assessed the role of alpha 2- and beta-adrenergic systems in mediating distress vocalizations. We found that the alpha 2 agonist, clonidine (33 and 67 micrograms/kg), reduced activity levels without affecting separation-induced coos. Only at 100 micrograms/kg were distress vocalizations reduced, and this was associated with behavioral sedation. In the same animals, morphine (0.1 and 0.25 mg/kg) selectively reduced distress vocalizations without affecting activity. Thus, the effects of clonidine, 100 micrograms/kg, appear to be due to non-specific sedation. We next assessed whether antagonism of beta-adrenergic receptors reduces separation distress. We administered propranolol over a wide dosage range to a different group of animals and found that a high dose (20 mg/kg) increased separation-induced coos while decreasing the activity levels. That such a high dose was necessary to affect coo vocalizations suggests that effects on vocalizations are due to non-specific effects of the drug. Unlike propranolol, morphine administered to these animals did not affect activity levels but did selectively reduce distress vocalizations. These findings suggest that alpha 2- and beta-adrenergic systems do not specifically mediate separation-induced coos in infant rhesus monkeys. However, as demonstrated in an earlier study, opiate systems have a prominent role.